Just as the number line can be extended to the left to include negative numbers, the $x$- and $y$-axes can also be extended to include negative values. This creates the coordinate plane, a system that can be used to describe the locations of points.

For example, point $B$ can be described by the ordered pair $(\text-4,1)$. The $x$-value of -4 tells us that the point is 4 units to the left of the $y$-axis. The $y$-value of 1 tells us that the point is 1 unit above the $x$-axis. Point $B$ is located in Quadrant II.

The same reasoning applies to the points $A$ and $C$. The $x$- and $y$-coordinates for point $A$ are positive, so $A$ is to the right of the $y$-axis and above the $x$-axis. Point $A$ is located in Quadrant I.

The $x$- and $y$-coordinates for point $C$ are negative, so $C$ is to the left of the $y$-axis and below the $x$-axis. Point $C$ is located in Quadrant III.

Quadrant IV contains points whose $x$-coordinates are positive and whose $y$-coordinates are negative.